【单选题】

Have you ever been startled by a loud noise or by someone appearing suddenly behind you on dark night or by a snake in the grassDo you remember the sudden, tight feeling in your chest, the way your heart started go pound, the way you seemed to breathe more rapidly Such reactions occurred because you were frighteneD、Your brain sent a signal to the special gland (腺) in your body caned the adrenal (肾上的) glanD、Other parts of your body were involved, too, but the adrenal gland has a very special function.

The adrenal gland is located on top of your kidney. Since you have two kidneys. You also have two adrenal glands.Each adrenal gland is divided into two parts: a cortex (外皮) and a medulla (髓质). The cortex forms the outer shell; the medulla forms the central core.Both the cortex and the medulla secrete (分泌) hormones, but they have different functions.
The cortex produces more than forty different chemical compounds and hormones.All these things are essential to sustain life. The medulla secretes adrenalin (also called epinephrine, the hormone that prepares the body for emergencies.)
When you are suddenly faced with a dangerous or stressful situation, part of your brain instantly sends a signal to the adrenal gland, and all sorts of things begin to your body. The medulla releases an extra amount of adrenalin into the bloodstream. The adrenalin is carried rapidly throughout the body, stimulating other glands and systems.
It is ______ that mainly helps you respond to a dangerous or stressful situation.
A、a signal sent by your brain B、the special function of the adrenal gland
C、all stimulated parts of your bodyD、All of these